Intramolecular Electron Transfer in Frozen Solvents: Charge Transfer and Local Triplet States Population Dynamics Revealed by Dual Phosphorescence.

In frozen solvents at 77 K, ultrafast (≤250 fs) photoinduced intramolecular electron transfer (ET) in bichromophoric donor-acceptor ([D-A]) diarylmethane lactones produces a covalently linked radical ion pair, 1[D•+-A•-]. Steady state and time-resolved luminescence measurements reveal that 1[D•+-A•-] decays to charge-separated (3[D•+-A•-]) and donor-centered ([3D*-A]) triplets, which display dual phosphorescence. 3[D•+-A•-] and [3D*-A] are formed in parallel via two intersystem crossing mechanisms: spin orbit charge transfer (SOCT) and hyperfine coupling (HFC), with solvent dependent branching ratio. The solvent drives the D-A alignment during the freezing process to adapt to increasing solvent polarity, producing inhomogeneous ground-state population distribution with solvent-dependent D-A exchange interaction, which plays a key role in partitioning into SOCT and HFC mechanisms. In polar glasses, a third phosphorescence band appears due to dissociative back ET in 3[D•+-A•-] resulting in excited open ring biradical.

Tuneable white fluorescence from intramolecular exciplexes.

Crystal violet lactone (CVL) in solution displays unusually broad (FWHM > 9100 cm(-1)) dual fluorescence with the characteristics of white light. The emission combines a blue CT band from a local chromophore with an orange CT band from an intramolecular exciplex formed adiabatically at appropriate medium polarity. The fluorescence spectrum can be controlled by solvent polarity to yield tuneable emission colours in a broad range of coordinates in the CIE chromaticity diagram including the white region. We show that such dual emission is a general property of CVL-like D-A structures built on sp(3) carbon atoms. The dependence of excited state energetics on molecular structure allows the prediction of width, shape and other parameters of the dual fluorescence spectrum, and so enables the engineering and customised design of white fluorophores. The photophysics-structure relationship found for CVL and its analogues can be generalized into a novel concept of white light generation by small molecules. These D-A systems are studied as a template basis for design and development of white fluorophores.